// ******************************************************************************
   //
   // Title:       Force Field X.
   // Description: Force Field X - Software for Molecular Biophysics.
   // Copyright:   Copyright (c) Michael J. Schnieders 2001-2025.
   //
   // This file is part of Force Field X.
   //
   // Force Field X is free software; you can redistribute it and/or modify it
  // under the terms of the GNU General Public License version 3 as published by
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  // Force Field X; if not, write to the Free Software Foundation, Inc., 59 Temple
  // Place, Suite 330, Boston, MA 02111-1307 USA
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  package ffx.algorithms.mc;
  
  import ffx.algorithms.AlgorithmListener;
  import ffx.algorithms.cli.DynamicsOptions;
  import ffx.algorithms.dynamics.MDVerbosity;
  import ffx.algorithms.dynamics.MDWriteAction;
  import ffx.algorithms.dynamics.MolecularDynamics;
  import ffx.algorithms.dynamics.MolecularDynamicsOpenMM;
  import ffx.numerics.Potential;
  import ffx.potential.MolecularAssembly;
  import ffx.utilities.Constants;
  
  import javax.annotation.Nullable;
  import java.io.File;
  import java.util.EnumSet;
  import java.util.logging.Level;
  import java.util.logging.Logger;
  
  import static java.lang.Math.abs;
  import static java.lang.Math.min;
  import static java.lang.String.format;
  
  /**
   * Use MD as a coordinate based MC move.
   *
   * @author Mallory R. Tollefson
   */
  public class MDMove implements MCMove {
  
    private static final Logger logger = Logger.getLogger(MDMove.class.getName());
  
    /**
     * THe MD instance that executes the move.
     */
    private final MolecularDynamics molecularDynamics;
    /**
     * Number of MD steps per move.
     */
    private final long mdSteps;
    /**
     * Time step in femtoseconds.
     */
    private final double timeStep;
    /**
     * Print interval in picoseconds.
     */
    private final double printInterval;
    /**
     * Temperature in Kelvin.
     */
    private final double temperature;
    /**
     * Snapshot appending interval in psec.
     */
    private final double saveInterval;
    /**
     * Potential to operate on.
     */
    private final Potential potential;
    /**
     * Number of MD moves.
     */
   private int mdMoveCounter = 0;
   /**
    * The total energy change for the current move.
    */
   private double energyChange;
   /**
    * Absolute energy drift over all moves.
    */
   private double energyDriftAbs;
   /**
    * Net energy drift over all moves.
    */
   private double energyDriftNet;
   /**
    * Kinetic energy at the start of the move.
    */
   private double initialKinetic;
   /**
    * Potential energy at the start of the move.
    */
   private double initialPotential;
   /**
    * Total energy at the start of the move.
    */
   private double initialTotal;
 
   private File dynRestartFile;
 
   /**
    * Constructor for MDMove.
    *
    * @param assembly        a {@link ffx.potential.MolecularAssembly} object.
    * @param potentialEnergy a {@link ffx.numerics.Potential} object.
    * @param listener        a {@link ffx.algorithms.AlgorithmListener} object.
    * @param dynamics        CLI object containing key MD information.
    * @param stepsPerCycle   Number of MD steps per MC cycle.
    * @param dynRestartFile  File to read restart from.
    */
   public MDMove(MolecularAssembly assembly, Potential potentialEnergy, AlgorithmListener listener,
                 DynamicsOptions dynamics, long stepsPerCycle, @Nullable File dynRestartFile) {
     this.potential = potentialEnergy;
     molecularDynamics = MolecularDynamics.dynamicsFactory(assembly, potentialEnergy, listener,
         dynamics.thermostat, dynamics.integrator);
     molecularDynamics.setAutomaticWriteouts(false);
 
     timeStep = dynamics.getDt();
     double dtPs = timeStep * Constants.FSEC_TO_PSEC;
     mdSteps = stepsPerCycle;
 
     molecularDynamics.setVerbosityLevel(MDVerbosity.QUIET);
     molecularDynamics.setObtainVelAcc(false);
     molecularDynamics.setRestartFrequency(dynamics.getCheckpoint());
     this.saveInterval = dynamics.getSnapshotInterval();
 
     double requestedPrint = dynamics.getReport();
     double maxPrintInterval = dtPs * mdSteps;
     // Log at least once/cycle.
     printInterval = min(requestedPrint, maxPrintInterval);
     this.temperature = dynamics.getTemperature();
 
     this.dynRestartFile = dynRestartFile;
     // Load the restart file if it exists.
     if (this.dynRestartFile != null && this.dynRestartFile.exists()) {
       molecularDynamics.setVerbosityLevel(MDVerbosity.SILENT);
       molecularDynamics.dynamic(1, timeStep, printInterval, saveInterval, temperature, false, dynRestartFile);
       collectEnergies();
       revertMove();
       molecularDynamics.setVerbosityLevel(MDVerbosity.QUIET);
     }
   }
 
   /**
    * Get the total energy change for the current move.
    *
    * @return Total energy change.
    */
   public double getEnergyChange() {
     return energyChange;
   }
 
   public double getInitialKinetic() {
     return initialKinetic;
   }
 
   public double getInitialPotential() {
     return initialPotential;
   }
 
   public double getInitialTotal() {
     return initialTotal;
   }
 
   /**
    * getKineticEnergy.
    *
    * @return a double.
    */
   public double getKineticEnergy() {
     return molecularDynamics.getKineticEnergy();
   }
 
   public MolecularDynamics getMD() {
     return molecularDynamics;
   }
 
   /**
    * getPotentialEnergy.
    *
    * @return a double.
    */
   public double getPotentialEnergy() {
     return molecularDynamics.getPotentialEnergy();
   }
 
   /**
    * {@inheritDoc}
    */
   @Override
   public void move() {
     move(MDVerbosity.QUIET);
   }
 
   /**
    * Performs an MDMove.
    *
    * @param verbosityLevel How verbose to be.
    */
   public void move(MDVerbosity verbosityLevel) {
     MDVerbosity origLevel = molecularDynamics.getVerbosityLevel();
     molecularDynamics.setVerbosityLevel(verbosityLevel);
     if (mdMoveCounter == 0 && dynRestartFile != null && dynRestartFile.exists()) {
       molecularDynamics.dynamic(mdSteps, timeStep, printInterval, saveInterval, temperature, false, dynRestartFile);
     } else {
       molecularDynamics.dynamic(mdSteps, timeStep, printInterval, saveInterval, temperature, true, null);
     }
     mdMoveCounter++;
     collectEnergies();
     energyChange = molecularDynamics.getTotalEnergy() - initialTotal;
 
     if (molecularDynamics instanceof MolecularDynamicsOpenMM && logger.isLoggable(Level.FINE)) {
       energyDriftNet += energyChange;
       energyDriftAbs += abs(energyChange);
       double energyDriftAverageNet = energyDriftNet / mdMoveCounter;
       double energyDriftAverageAbs = energyDriftAbs / mdMoveCounter;
       logger.fine(format(" Mean signed/unsigned energy drift:                   %8.4f/%8.4f",
           energyDriftAverageNet, energyDriftAverageAbs));
 
       double dt = molecularDynamics.getTimeStep();
       int intervalSteps = molecularDynamics.getIntervalSteps();
       int nAtoms = potential.getNumberOfVariables() / 3;
       // TODO: Determine if the *1000 factor is an old artifact of MolecularDynamicsOpenMM being the
       // one thing which (used to) store dt in fsec.
       double normalizedEnergyDriftNet = (energyDriftAverageNet / (dt * intervalSteps * nAtoms)) * 1000;
       double normalizedEnergyDriftAbs = (energyDriftAverageAbs / (dt * intervalSteps * nAtoms)) * 1000;
       logger.fine(format(" Mean singed/unsigned energy drift per psec per atom: %8.4f/%8.4f\n",
           normalizedEnergyDriftNet, normalizedEnergyDriftAbs));
     }
     molecularDynamics.setVerbosityLevel(origLevel);
   }
 
   /**
    * {@inheritDoc}
    */
   @Override
   public void revertMove() {
     try {
       molecularDynamics.revertState();
     } catch (Exception ex) {
       logger.severe(" The MD state could not be reverted.");
     }
   }
 
   public void setMDIntervalSteps(int intervalSteps) {
     molecularDynamics.setIntervalSteps(intervalSteps);
   }
 
   /**
    * Write restart and trajectory files if the provided step matches the frequency.
    *
    * @param mdStep      MD step (not MC cycle number) to write files (if any) for.
    * @param trySnapshot If false, do not write snapshot even if the time step is correct.
    * @param tryRestart  If false, do not write a restart file even if the time step is correct.
    * @return Returns the write actions.
    */
   public EnumSet<MDWriteAction> writeFilesForStep(long mdStep, boolean trySnapshot, boolean tryRestart) {
     return molecularDynamics.writeFilesForStep(mdStep, trySnapshot, tryRestart);
   }
 
   private void collectEnergies() {
     initialTotal = molecularDynamics.getInitialTotalEnergy();
     initialKinetic = molecularDynamics.getInitialKineticEnergy();
     initialPotential = molecularDynamics.getInitialPotentialEnergy();
     // If total energy is non-tiny, assert that kinetic + potential = total to within tolerance.
     assert abs(initialTotal) < 1.0E-3
         || abs((initialKinetic + initialPotential - initialTotal) / initialTotal) < 1.0E-7;
   }
 }